Radiation sensitive plates suitable for lithographic printing are well known. Such plates typically consist of a substrate such as aluminum that may be grained and/or anodized, or of zinc, magnesium, copper or steel or a combination such as a bimetal or laminate, and a layer of radiation sensitive material deposited on the substrate.
Image-wise exposing the radiation sensitive layer to actinic radiation through a transparency causes the solubility of the radiation-exposed areas to change relative to that of the unexposed areas. Upon treatment of the exposed plate with a suitable developer, the more soluble areas can be readily removed to reveal the underlying substrate and leave an image on the substrate constituted by the less soluble areas. The areas of the substrate revealed upon development constitute the non-image areas.
A type of radiation sensitive materials known as photopolymers become less soluble after exposure to radiation and therefore a negative transparency is used in the exposure. In this case, it is the non-radiation-exposed areas that are removed upon development and the radiation-exposed areas that remain on the substrate form the image. Examples of suitable photopolymers include diazo resins chromium-sensitized colloids, diazonium or azide-sensitized resins or polymers bearing such groups. Plates having a radiation sensitive layer based on such materials are known as negative-working plates.
Radiation sensitive materials such as those based on orthoquinone diazides become more soluble after exposure to radiation and therefore a positive transparency is used in the exposure. In this case, it is the radiation-exposed areas that are removed by development and the non-radiation-exposed areas that remain on the substrate form the image. Plates having such radiation sensitive layers are known as positive-working plates.
The life, in terms of the number of copies it can produce, of a printing plate can often be increased by baking the plates, i.e. "burning-in" the image areas, provided, of course, the material of the image areas is suitable. "Burning-in" is a well-established practice in the art of producing lithographic printing plates from radiation-sensitive plates ("burning-in" is most commonly carried out with positive-working plates). The "burning-in" causes extensive crosslinking to occur in the polymeric structure of the material comprising the image area. The limiting temperature and time of the "burning-in" is that at which the aluminum anneals, resulting in a loss of strength required for a printing plate.
U.K. Patent 669,412 discloses the burning-in of images based on naphthoquinone diazides. In accordance with the teachings of this patent, a radiation sensitive plate including a layer of the diazide is image-wise exposed, developed with an alkaline solution to remove selectively those areas of the layer exposed to radiation, and then placed in an oven to heat the image constituted by those areas of the layer that were not exposed to radiation. Thereafter, it is necessary to treat the plate with an alkaline solution again in order to remove contaminating residues from the plate and make the plate ready for printing.
In many cases, the image areas to be heated may be reinforced by incorporating reinforcing material in the radiation sensitive layer and/or by applying the reinforcing material in the form of a reinforcing lacquer to the image areas after development. Novolak resins and/or resol resins are examples of commonly used reinforcing materials. However, as disclosed in U.K. Patent 1,154,749, heating at a temperature sufficient to harden resin-reinforced image areas causes those areas of the substrate revealed on development to be at least partially covered with a contaminating layer which is ink accepting and which would therefore cause scumming and yield a soiled background during printing. This layer must therefore be removed before printing is initiated and this is achieved in accordance with the teachings of the patent by treating the plate with aqueous alkaline solution.
U.S. Pat. No. 4,294,910 discloses the use of various aqueous compositions known as "gumming" or "pre-bake" solutions to avoid problems resulting from the burning-in process. Such solutions contain materials such as sodium dodecyl phenoxybenzene disulfonate and the sodium salts of alkylated naphthalene sulfonic acid, sulfonated alkyl diphenyl oxide, methylene dinaphthalene disulfonic acid, etc.
U.S. Pat. No. 4,786,581 discloses the use of "gumming" solutions for protecting plates during the burning-in process; these aqueous solutions contain a hydrophilic polymer component and an organic acid component. The organic acid component (or water-soluble salt thereof) contains di- or greater acid functionality and encompasses the benzene carboxylic acids, sulfonic acids and phosphonic acids including alkane phosphonic acids. In contradistinction to the materials recited in the '581 patent, the present invention does not require the use of a hydrophilic polymer. U.S. Pat. No. 4,885,230 discloses the use of water-soluble homopolymers and copolymers of monomers such as styrene sulfonic acid, vinyl phosphonic acid, etc. and water-soluble salts thereof. However, the compositions disclosed in this patent do not include polymers containing strong acid functionalities and their salts.
The contaminating layer produced as the result of the burning-in process is not, as a rule, discernible to the naked eye and it is difficult to ensure that all the contamination has been removed. Moreover, in the case of those substrate surfaces that are porous, as is the case of an anodized aluminum plate, contamination may be present in the pores as a result of residual material left behind from the development process. Such contamination is likely to even cause scumming during long printing runs as the substrate surface is gradually worn away. The alternative of redeveloping the plates after burning-in in order to remove the contaminating layer is costly and inconvenient since the plates have to be returned to the plate fabrication facility after they have been removed from the oven.
In view of the difficulties associated with the removal of the contamination which is produced by the burning-in procedure, it is desirable to prevent such contamination from occurring in the first instance. Also, it has been found that the contamination apparently arises as a result of some component of the image material subliming from the image areas during heating and subsequently being redeposited on the areas of the substrate revealed on development. Even plates that contain no substances that could generate contamination during heating nevertheless have become contaminated by deposition of contaminating material previously deposited on the internal surfaces of the burning-in oven as a result of prior usage. Thus, an effective pre-bake gum formulation is essential for success in the burning-in process.
It is important to consider the requirements for an effective pre-bake gum solution. The components of the solution must act to clean or even seal the porous aluminum oxide surface. Indeed, such cleaning must occur with precision around the fine image structure of the printing plate. On the other hand, the components of the pre-bake gum must also not attack the coating that constitutes the image area. Such components must not only avoid penetration to the base or near to the base, but must also avoid the creation of serious irregularities in the coating surface. Deficiencies occurring during the pre-bake process can bring about coating failure under the stress of press conditions.
The aqueous composition of the present invention provides superior pre-bake protection while allowing for a wide latitude of coating thickness. The superior properties in comparison to the prior art compositions arise out of a better coating integrity during lay-down on the plate surface as well as during pyrolysis (i.e. during "burning-in"). Furthermore, the anionic surfactant component of the composition of this invention has a viscosity depressant property and also has a unique wetting activity that increases markedly (i.e. 10-30 fold) with temperature.